Lighting dimmers mounted in wall boxes often require, by building safety code, the ability to completely disconnect the power that is provided to the lighting load (or other type of dimmable load) when servicing the load. For example, servicing the load can be changing a burned-out light bulb or florescent tube. Standard mechanical light switches accomplish this by definition, because such switches mechanically open the electrical circuit and prevent current from flowing.
Electronic dimmers operate by essentially restricting the average current flow through the load by means of controlling the conduction of the load current using a solid-state device such a triac. The longer the triac is allowed to conduct in each AC cycle, the more average current is provided to the load. The “OFF” state is when the triac is not allowed to conduct at all. Even though a light bulb will appear to be completely off in this state, there is measurable leakage current through the triac that governmental and/or other safety agencies deem to be potentially dangerous. Therefore, dimmers may be required to have a mechanical switching means to open the circuit for purposes of servicing the load, and this is referred to as an “air-gap switch”. Various means have been previously devised to provide dimmers with the required air-gap functionality.
One known air-gap mechanism uses a plastic pull-down switch that protrudes downwardly from the bottom of the switch faceplate. This pull-down switch is oriented parallel with and against the wall. In the normal “ON” position, the clear plastic air-gap actuator is barely visible below the faceplate. The air-gap switch is activated to turn the load off by pulling down on the actuator. The shaft operates a linearly-actuated mechanical air-gap switch. Some disadvantages of this general design are: 1) the actuator is visible and unattractive because it protrudes from the bottom of the faceplate, and 2) it may require notching out the back of faceplates that a homeowner or decorator may wish to attach to the dimmer in order to accommodate the shaft of the air-gap switch actuator. In the instance of a metal plate, it may not even be possible to modify a particular faceplate to work with this type of air-gap switch.
With the foregoing limitations in mind, other dimmer manufacturers have chosen to incorporate the air-gap switching function within the rectangular switch plate opening, which solves the problem of interference with the faceplate. They either incorporate the switch within a narrow frame that surrounds the switch keycap, or they make the keycap smaller than the switch opening to accommodate an additional switch for the air-gap function. One disadvantage of these designs is the air-gap switch is visible and interferes with the aesthetics of the design, at best, or is downright ugly, at worst.
Other manufacturers have tried to solve the aesthetics problem by including a processor-controlled relay within the dimmer that automatically provides the air-gap function. The processor can open the load circuit with a relay every time the dimmer is either switched off or dimmed until the minimum level is reached (which is by definition off). This switch configuration has no need for any type of externally actuated air-gap switch since its air-gap function is actuated by the switch keycap itself using the relay.
This processor and relay method works fine if the microcontroller in the dimmer is always active, which means the dimmer is always receiving AC power. The dimmer always receives power in cases when a neutral or ground wire is available in the wall box in which the dimmer is installed. However, it is common to have wiring situations where a neutral wire is not available in the wall box. In these situations, the dimmer is not powered in parallel with the hot and neutral AC wires, but in series with the hot and load wires. For series connections, a special type of power supply is needed to power the dimmer. In essence, the “load-line-powered” dimmer's power supply steals some of the current from the hot lead to power its own circuitry, while its power supply return path is actually through the load.
Of course, if this load-line-powered dimmer's power supply return path is opened for any reason, then the dimmer is shut off just as though a power switch shut off the dimmer's circuitry. Once the dimmer's microcontroller is shut off, it has no way to close the relay again. Yet, if the relay is not ever closed again, the microcontroller will never receive power to allow itself to operate. This places the microcontroller in the situation of needing power to close the switch but not having power to operate itself. This is why a relay-air-gap-style dimmer was designed to only be installed in situations where a neutral is available. A load-line-powered, relay-air-gap version of such a dimmer is not provided, because the circuit topology simply cannot be used in a series-connected configuration.